In a number of chemical processes, i.e. chlorination of organic materials, hydrochloric acid is produced as a by-product. This by-product finds limited use in industrial processes and is also often discarded in water streams near production facilities. Occasionally, the hydrochloric acid is neutralized and the salt by-product disposed off by dumping, piling or burying. Utilization of the by-product at the production site is often limited; transporting the by-product is not economical, neutralization and disposal create additional costs which burden the production cost of the original chemicals.
Manufacturers of hydrochloric acid by-product often recover the chlorine from hydrochloric acid for the purpose of recycling valuable chlorine while reducing or even eliminating environmental pollution.
As an illustration of the magnitude of this potential problem, Chemical & Engineering News, Feb. 21, 1977 reports that in 1976 chlorinated hydrocarbons consumed approximately 55% of the U.S. chlorine production of 10.5 million tons, including 20% of the production of vinyl chloride. Vinyl chloride production alone therefore accounted for as much as 1 million tons of hydrochloric acid by-product per year. If converted back into chlorine this volume would represent as much as 1 million tons of chlorine.
In some other applications, such as water chlorination, transportation and handling of compressed chlorine may create unnecessary hazards. Therefore, a system using a much less hazardous chemical such as dilute hydrochloric acid, could find extensive applications in small on-site chlorine-consuming water treatment facilities. As an illustration, Chemical & Engineering News, Feb. 21, 1977 estimated that 5% of the 1976 U.S. production of chlorine is used for water treatment. Therefore, a total volume of 0.5 million tons of chlorine is transported in small pressurized containers, each representing a handling hazard. This hazard could be avoided if chlorine could be produced on site.
These examples are meant to illustrate the large demand for chlorine while considerable amounts of by-product hydrochloric acid are produced, which could become the raw material for a great fraction of the chlorine demand, if economical recovery processes are available. Various methods have been developed for producing chlorine from hydrochloric acid. These methods can be classified into two major groups: chemical methods and electrochemical methods. The best known chemical methods consist of the catalytic oxidation of hydrochloric acid by oxygen or the direct oxidation of hydrochloric acid by nitrogen dioxide. The best known electrochemical methods consist of the direct electrolysis of hydrochloric acid into chlorine or the electrolysis of metal salts derived from aqueous hydrochloric acid.
However, of these four methods, the direct electrolysis process is generally preferred because of simplicity of equipment and operation and relatively low investment costs. The best known of these direct electrolysis processes is of the filter-press type, disclosed in U.S. Pat. No. 3,236,760 to G. Messner and 3,242,065 to O. deNora and G. Messner, making use of bipolar electrolytic cells with vertical electrodes and acid resistant diaphragms as separators. The cell operates at 80.degree.-90.degree. C. and uses graphite or platinized titanium electrodes. For an effluent hydrochloric acid concentration of 18-19 weight percent, which requires a concentrated hydrochloric acid feed containing approximately 33 weight percent of HCl the single cell voltage is 2.3 volts, corresponding to a DC power consumption of 1750 kilowatt-hour for 2,000 pounds of chlorine or an efficiency of 0.83 Kw-hr/lb of chlorine. For separation of gaseous chlorine from the gaseous hydrogen, a diaphragm is used, which has to be renewed from time to time. The single elements are assembled together between end-plates, clamped by means of hand-wheel operated capstan screws. These cells use graphite lumps which can be the source of local heating and inadequate electrical contact, resulting in increased power consumption. Since the specific power consumption, kw-hr/lb of chlorine, represents a large contribution to chlorine production costs, many attempts have been made to reduce power requirements.
For example, in U.S. Pat. No. 3,117,066 to W. Juda, hydrogen produced in the hydrochloric acid electrolytic cell is recombined with oxygen from air to generate power, thus reducing the chlorine production process power requirement by as much as 33%.
Most of the development work of electrochemical processes conducted to date, such as those described in the aforementioned prior art patents, has been associated with stationary, flow-by or flow-through electrodes. Various types of such electrodes have been utilized. The best known are probably sintered metal electrodes, vacuum deposited electrodes on organic supports, metallic diffusion electrodes (Ag/Pd, Pd), flexible organically bonded compressed powder electrodes (as had been applied in ion exchange membrane fuel cells) or graphite electrodes. All of these electrodes present characteristic interfaces between the conducting electrolyte (liquid or solid) and the gas phase (oxidant or reducing fuel). Reaction rates of these electrodes are associated with mass transport processes through thin films covering discrete regions in the electrode structure.
It is apparent that for such electrodes, only the fraction of the total electrode surface which is in contact with the electrolyte is available for the electrochemical reaction. Since the electrolyte offers a high resistance of mass transport to the electrode surface surrounded by the electrolyte, the reaction zone is limited to discrete zones in individual electrode pores.
Thus, where stationary or flow-by or flow-through porous electrodes are employed in electrochemical cells for conversion of hydrochloric acid into chlorine, such cells become attacked if local depletion of hydrochloric acid is occuring, which may render the cells uneconomical to operate in view of corrosion and mass transfer limitations. Such corrosion has been reported in U.S. Pat. No. 3,242,065. Furthermore, local heating effects require an excess flow of feed acid through the electrolyzer, as reported in U.S. Pat. No. 3,855,104 to G. Messner. These excess flow rates reduce the efficiency of conversion of hydrochloric acid to chlorine and only deplete the acid concentration to 18% HCl from a feed concentration of 33% HCl.
Accordingly, it is clear that prior art procedures for the conversion of hydrochloric acid into chlorine are not entirely satisfactory in that they consumed considerable amounts of energy, depend on filter-press arrangements requiring tightly fitting designs and require concentrated feeds to avoid corrosion and excessive energy consumption.